1. Field of the Invention
The present invention relates generally to low-noise electronic systems.
2. Description of the Related Art
Electrical noise is a ubiquitous phenomenon in electronic devices and it typically sets a lower bound on the sensitivity of electronic systems. Electrical noise generally includes thermal noise and shot noise. Thermal noise is generated by random thermal motion of charged particles and is associated with thermodynamic energy exchanges that maintain thermal equilibrium between a circuit and its surroundings. In contrast, shot noise is generated by the random passage of discrete current carriers across barriers or discontinuities (e.g., semiconductor junctions).
Two other noise components originate in low-frequency conductance fluctuations within electrical devices. The first component exhibits a Lorentzian frequency dependence in its power spectral density. It is referred to as G-R noise because it originates from fluctuations in the number of free electrons in device conduction bands that are caused by generation and recombination processes between the bands and interacting traps. The second component exhibits a 1/f.sup..alpha. (0.4&lt;.alpha.&lt;1.2) power spectral density. Although its generation is not well understood, a multitude of mechanisms appear to generate it including superposition of G-R spectra with different characteristic times and weights.
The performance of active electronic circuits is degraded by the presence of noise. In a first exemplary degradation, the noise figure of low-noise amplifiers (LNA) is increased. Receiver noise figure is similarly increased because it is primarily determined by the noise figure of the receiver's LNA. Excess noise in LNA's typically manifests itself in device signal fluctuations (e.g., current fluctuations in the gate and drain of field-effect transistors). Oscillator phase noise is increased in a second exemplary degradation. Phase noise in the output signal of oscillators generally results from upconversion of low frequency noise. In a third exemplary degradation, phase noise is added to the output of clock circuits which lowers the performance of systems associated with the clock. For example, phase noise in sampling clocks decreases the dynamic range of analog-to-digital converters.
Conventional methods for reducing noise signals in electronic circuits have generally included the steps of, a) designing electronic device structures with reduced surface area, b) employing materials and processes with favorable carrier generation/recombination parameters and c) selecting active devices that exhibit low excess noise characteristics.
Regardless of the nature of an active device, excess noise is physically associated with statistical processes (e.g., carrier generation and recombination) at various device locations (e.g., surface/passivation interfaces and bulk interfaces such as junctions and heterojunctions). Whatever the specific model adopted to interpret excess noise frequency dependence, conductance fluctuations (which produce measurable voltage fluctuations) are caused by spontaneous emission of atomic carriers. In contrast to stimulated emission which is induced by the presence of radiant energy of like frequency and wavelength, spontaneous emission in a quantum mechanical system is radiation that is emitted when the internal system energy spontaneously drops from an excited state to a lower state without regard to the simultaneous presence of similar radiation.
A reference on spontaneous emission (Yablonovitch, Eli, "Inhibited Spontaneous Emission in Solid-State Physics and Electronics", The American Physical Society, Vol. 58, No. 20, May 18, 1987, pp. 2059-2062), points out that it is neither feasible nor desirable to eliminate spontaneous emission entirely if a function of a semiconductor structure (e.g., a laser or a solar cell) is the emission or absorption of light. Rather, the goal in those cases is to restrict spontaneous emission to those electromagnetic modes that are absolutely necessary.
This reference observes that periodic spatial modulation (e.g., in distributed-feedback lasers and interference coatings for wave optics) opens up a forbidden gap in the electromagnetic dispersion relation. For example, three-dimensional spatial periodicity of .lambda./2 in the refractive index can result in a forbidden gap in the electromagnetic spectrum near the wavelength .lambda.. If the electromagnetic band gap overlaps an electronic band edge, then electron-hole radiative recombination (hence, spontaneous emission) will be severely inhibited.
The reference concludes that inhibited spontaneous emission is a real possibility in semiconductor lasers but requires further materials development before the benefits are fully realized. With respect to heterojunction bipolar transistors, the reference teaches minimizing of transistor electron-hole recombination with consequent enhancement of transistor current gain. Because of conflicting requirements (e.g., high base doping to obtain low series resistance and high speed operation), the reference concludes that this application of inhibited spontaneous radiation would be limited to transistors of moderate base doping.
A second reference (Sigalas, M. M., et al., "Metallic Photonic Band-gap Materials", The American Physical Society, Vol. 52, No. 16, October 1995, pp. 11744-11751) compares metallic photonic band-gap structures to dielectric photonic bandgap crystals (PBC's). It calculates transmission and absorption characteristics of electromagnetic waves for two-dimensional and three-dimensional periodic structures. In two-dimensional metallic structures, it was determined that propagating modes of s-polarized waves are interrupted by band gaps (behavior similar to that of dielectric PBC's) while p-polarized waves exhibit a cutoff frequency below which propagating modes are severely attenuated. Three-dimensional metallic structures with isolated metallic scatterers were found to behave similar to dielectric PBC's but continuous networks of metallic scatterers were found to have no propagating modes below a cutoff frequency for both s-polarized and p-polarized waves.
A third reference (Sievenpiper, M. M., et al., "3D Wire Mesh Photonic Crystals", The American Physical Society, Vol. 76, No. 14, April 1996, pp. 2480-2483) describes three dimensional wire mesh structures having a geometry similar to covalently bonded diamond. Similar to dielectric PBC's, the frequency and wave vector dispersion show forbidden bands at frequencies .nu..sub.o corresponding to the lattice spacing. In addition, they have a forbidden band extending from zero frequency to .about.1/2 .nu..sub.o.
As defined in a fourth reference (Brown, E. R., et al., "Radiation Properties of a Planar Antenna on a Photonic-Crystal Substrate", Journal of the Optical Society of America, Vol. 10, No. 2, February 1993, pp. 404-407), a photonic bandgap crystal (PBC) is a periodic structure that exhibits a forbidden band of frequencies (i.e., a photonic bandgap) in its electromagnetic dispersion.
This latter reference introduces PBC's as a substrate material for planar antennas and describes an experimental "bow tie" microstrip antenna that was fabricated by adhering copper tape to surfaces of a PBC. The PBC had a bandgap between 13 and 16 GHz and was fabricated by drilling holes in an epoxy-based dielectric having a dielectric constant of .about.13. The radiation performance of this experimental antenna was compared with that of a conventional antenna that was fabricated with a solid substrate of the same dielectric material. Measured radiation patterns of the second antenna indicated that it radiated primarily into its substrate with a lesser, useful radiation into the air. In contrast, measured radiation patterns of the first antenna indicated that its radiation was predominately confined as useful radiation into the air. In a summary of the experimental antenna's performance, it was stated that the PBC substrate expels radiation by Bragg scattering and, consequently, radiation is neither trapped in the substrate nor reflected back at such a phase as to lower the resistance of the antenna's driving point.
Although these references describe various PBC structures and teach the use of a PBC in expelling radiation from a substrate, they fail to provide any guidance to noise-reduction in active circuits (i.e., circuits having components which perform dynamic functions such as amplification, oscillation and signal modification).